「閉める」 (shimeru) means "to close" a door, window, or lid. It's a transitive verb, meaning someone actively closes something. For example, "ドアを閉めます" (Doa o shimemasu) means "I close the door." You are performing the action of closing.

「閉(し)める」は、ドアや窓(まど)などを「閉(と)じる」ことです。誰(だれ)かがその動作(どうさ)をします。ドアを閉(し)めます。